Such a device and such a method are known from DE 100 51 462 A1. The known device comprises an x-ray source and an x-ray detector, which together move around an object to be investigated. The projection images recorded by the x-ray detector are routed to an image processor which corrects the beam hardening. In this case the image processor executes a post-reconstructive correction process. Within the framework of the post-reconstructive process, the image processor reconstructs volume images from the object to be investigated. The term volume images used here and below should be taken to be both three-dimensional and also two-dimensional cross-sectional images. A re-projection is then performed with only those pixels for which the image value lies above a predetermined threshold being taken into account. This enables the processing effort involved in re-projection to be reduced.
Furthermore a method and a device are known from DE 195 23 090 C1 which are used to correct image errors caused by x-ray scattering. With the known device and the known method scatter distribution is modeled on the basis of a physical model for the x-ray scatter and with the aid of the model for the x-ray scattering x-ray scatter amounts falling on the individual pixels of the detector are determined. The x-ray scatter amounts thus determined are then subtracted from the image values of the uncorrected projection images.
Both the beam hardening and also the x-ray scattering are non-linear effects which result in distortion of the reconstructed volume images when the volume images are reconstructed from the projection images. These image artifacts are especially striking in the form of bar artifacts or shadow artifacts in the soft tissue between heavily absorbent bone structures. These image artifacts can significantly adversely affect the quantitative accuracy of the volume images and lead to incorrect diagnoses.
The introduction of computer tomography devices with multi-line detectors and flat-panel detectors has meant that x-ray scatter correction through suitable image processing has become increasingly important. This is because suppression of x-ray scatter by masking it out is very expensive. Correcting these interference effects by suitable image processing is also expensive however. To restrict the processing effort involved in correcting x-ray scatter, simplified scatter models are usually employed, but such models are prone to systematic errors. The image errors caused by x-ray scatter in the reconstructed volume images can thus mostly not be entirely eliminated.
The situation is similar with the correction of beam hardening. It is possible with a known chemical composition of the object to be investigated to perform a precise correction of the hardening. However as a rule the precise chemical composition of the object to be investigated, especially the chemical composition of the bones in the human body is not known in detail. Thus, even after the correction of beam hardening certain shadow artifacts are left behind. A typical example of such effects known to the person skilled in the art is shadow effects between the petrous bones in the anterior cranial fossa.
A further source of errors can be an incorrect scaling of the projection images. Because of the linearity of the reconstruction of the volume image from the projection data it is not necessary as a rule to correctly scale the projection data beforehand, since the volume image can still also be re-scaled after the reconstruction. However this no longer applies if non-linear effects such as spectral hardening or x-ray scattering and their corrections are taken into account in the creation of the volume image. Thus an incorrect scaling of the projection images of for example 10% can lead to an error of significantly more than 10% in the x-ray scatter correction.